Method of preventing oxidation of oils in electrical equipment



United States Patent 2,744,038 METHOD OF PREVENTING OXIDATION OF OILS 1N ELECTRICAL EQUIPMENT John Harry Osvald Lindhe, Nynashamn, Sweden, assiguor to Aktiebolaget Nynas Petroleum, Nyuashamn, Sweden, a company .of Sweden N0 Drawing. Application July 10, 1952, Serial No. 298,199 4 Claims. (Cl. 148-617) This invention relates to a method of preventing oxidation of oil in electrical equipment containing a filling of oil, e. g. transformers, cables, condensers and the like, where the oil comes in contact with surfaces of metal, e. g. copper or iron, which catalytically accelerate the oxidation of the oil.

It has been proposed to counteract this oxidation of the oil which causes great inconveniences by decomposition of the oil and formation of sludge therein, by addition of oxidation inhibitors, usually compounds of the phenol type, whereby a considerable improvement in this respect is obtained.

Further it has been proposed to treat the metal surfaces of the apparatus with which the oil may come in contact, with a mineral oilsolution of phosphorous compounds which by reaction with the metal surfaces render them catalytically inactive towards oxidizable hydrocarbon oils (cf. applicants copending U. S. patent application Serial No. 221,337 of April 16, 1951 now Patent No. 2,643,962, granted June 30, 1953) and also in this manner oxidation of the oil subsequently supplied to the apparatus is considerably reduced.

The present invention is based on the surprising dis- I covery that it is possible, by a combination of these two measures, to obtain a considerably higher effect than could be expected, whereby the oil is rendered extremely resistant to oxidation.

In accordance therewith the method of the present invention consists therein that the apparatus is treated with a mineral oil solution containing a phosphorus compound selected from the class consisting of trialkylphosphites and trialkylthiophosphites which react with the metal surfaces forming catalytically inactive coatings thereon whereupon the trialkylphosphite or trialkylthiophosphite-containing oil is removed from the apparatus and oil to which has been added an oxidation inhibitor, preferably an alkylated phenol, is filled into the apparatus.

As examples of suitable phosphorous compounds for the treatment according to the invention there may especially be mentioned triisobutylphosphite and triisoamylphosphite and the corresponding triisothiophosphites. A preferred oxidation inhibitor is triisobutylphenol.

The quantity of phosphorous compound added to the oil, calculated as phosphorous in percent of the weight of the oil, may e. g. lie within the ranges 0.0002% to 0.5%, suitably between 0.002% and 0.1%, and preferably between 0.02% and 0.05%

The treatment of the apparatus with this solution may be carried out during /2 to 10 hours, preferably between 1 and 5 hours, at a temperature within the ranges 50 to 200 0., preferably between 80 and 120 C.

The amount of alkylated phenol compound added to the oil as inhibitor, e. g. triisobutylphenol, may e. g. lie within the range of 0.5 to 1.0%, or preferably between 0.2% and 0.3%, calculated on the weight of the oil.

In order to illustrate the effect which is obtained according to the invention the following comparative tests may be mentioned, which were carried out with an apparatus with copper surfaces, the oxidation tests being carried out according to the so called Anderson-Aseamethod, described in the Journal of the Institute of Petroleum, London, 1946, pages 412-423.

Carried out with-- active catalysts, e. g. the untreated metal surfaces of the apparatus, and transformer oil without inhibitor.

2 active catalysts and transformer oil to which had been added 0.2% by weight of triisobutylphenol. 3 active catalysts which had been treated 1 hour at C Oxidation test according to the Anderson-Asea-method After oxidation at 100 C. during 100 hours with oxygen the oil from test No. 4 still had the original, light yellow colour, while the other oils, after treatment in the same manner, had a deep red-brown colour.

Similar results were obtained when treating an apparatus with iron surfaces and also when using the other phosphorous compounds indicated above.

I claim:

1. A method of rendering copper and iron surfaces of electrical equipment catalytically inactive towards oxidizable hydrocarbon mineral oils, which comprises treating the said metal surfaces with a mineral oil containing dissolved therein from about 0.0002% to 0.5% of a phosphorous compound, calculated as phosphorous in per cent by weight of oil and selected from the class consisting of triisobutylphosphite, triisoarnylphosphite and the correponding triisothiophosphites, for a period of from about 1 to 5 hours and at a temperature between about 50 and 200 C., to form a phosphorous-containing coating on the metal surfaces, removing the phosphorous compound-containing oil from the apparatus and supplying an oil containing an alkylated phenol as inhibitor to the apparatus.

2. The method of preventing the oxidization of mineral oils in electrical equipment having inner contacting surfaces of iron and copper, which comprises treating said surfaces with a mineral oil containing dissolved therein from about 0.0002 to 0.5% by weight of a phosphorous compound, selected from the class consisting of triisobutylphosphite, triisoamylphosphite and the corresponding triisothiophosphites, at a temperature of from about 50 to 200 C. for from about /2 to 10 hours to form a coating of a phosphorous compound on the metal surface, removing the mineral oil containing the dissolved phosphorous compound and introducing into the so-coated electrical equipment a mineral oil containing dissolved therein a small amount of an alkylated phenol as an oxidation inhibitor.

3. The process of claim 2 wherein the alkylated phenol is triisobutylphenol.

4. The process of claim 2 wherein after the coating step a mineral oil is introduced into the equipment containing dissolved therein from about 0.5 to 1% of triisobutylphenol.

References Cited in the file of this patent UNITED STATES PATENTS 1,234,862 Brown July 31, 1917 1,344,372 Bains June 22, 1920 2,101,632 Weinrich Dec. 7, 1937 2,224,695 Prutton Dec. 10, 1940 2,225,533 Dewey Dec. 17, 1940 

1. A METHOD OF RENDERING COPPER AND IRON SURFACES OF ELECTRICAL EQUIPMENT CATALYTICALLY INACTIVE TOWARDS OXIDIZABLE HYDROCARBON MINERAL OILS, WHICH COMPRISES TREATING THE SAID METAL SURFACES WITH A MINERAL OIL COMPRISING DISSOLVED THEREIN FROM ABOUT 0.0002% TO 0.5% OF A PHOSPHOROUS COMPOUND, CALCULATED AS PHOSPHOROUS IN PER CENT BY WEIGHT OF OIL AND SELECTED FROM THE CLASS CONSISTING OF TRIISOBUTYLPHOSPHITE, TRIISOAMYLPHOSPHITE AND THE CORREPONDING TRIISOTHIOPHOSPHITES, FOR A PERIOD OF FROM ABOUT 1 TO 5 HOURS AND AT A TEMPERATURE BETWEEN ABOUT 50 AND 200* C., TO FORM A PHOSPHOROUS-CONTAINING COATING ON THE METAL SURFACES, REMOVING THE PHOSPHOROUS COMPOUND-CONTAINING OIL FROM THE APPARATUS AND SUPPLYING AN OIL CONTAINING AN ALKYLATED PHENOL AS INHIBITOR TO THE APPARATUS. 